Pilot line isolating valve

This waterside overpressure safety valve was operated by a spring-loaded pilot valve which, for test purposes, could be isolated from the water pressure in the steam drum by a valve called the PLIV - the pilot line isolating valve. During normal operation the PLIV was locked open with a large padlock. 

The only way to avoid the risk of hogging the drum was to have someone next to the Pilot Line Isolating Valve, the PLIV, to shut it immediately after the main waterside overpressure safety valve had opened. By closing the PLIV, the Pilot Valve would close, making the Safety Valve close also. The PLIV was a small handwheel-operated valve directly under the main steam drum, which was not normally readily... 

The main steam and feedwater lines is equipped with three system-fluid-operated isolation valves, one of every three being of a different design from that of the other two valves. Closure of the main steam valves to isolate the containment is based on the fail-safe principle employing solenoid pilot valves or through passive actuation. 

Each steam line has two containment isolation valves, one inside and one outside the containment barrier. The isolation valves are spring-loaded pneumatic piston-operated globe valves designed to fail closed on loss of pneumatic pressure or loss of power to the pilot valves. Each valve has an air accumulator to assist in the closiue of the valve upon loss of the air supply, electrical power to the pilot valves, and failure of the loaded spring. Each valve has an independent position switch initiating a signal into the reactor protection system scram trip circuit when the valve closes. 

On French plants, cold overpressure protection is provided by pilot operated safety valves on the pressurizer when the RHR system is isolated. When the RHR system is operating, the protection is provided by pilot operated safety valves on the discharge lines from the RHR pumps. All these valves are qualified to operate in water conditions. In addition, specific requirements have been included in the Technical Specifications. 

The RPV is provided with a pressure relief system which consists of 12 safety (relief) valves connected evenly onto the four steam lines, with blowdown pipes leading down into the condensation pool. The safety (relief) valves are own medium operated valves, each being controlled by two pilot valves, one pressure activated and one electrically controlled this means that actuation can be initiated in a controlled way by pressure monitoring equipment, to avoid over pressurization or to achieve depressurization. In addition, control valves are provided downstream two of the safety valves, in order to enable proper pressure control of the reactor also in the event of isolation (loss of the turbine condenser function). 

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